A remarkable trait of all organisms is their ability to adjust to the changes in the surrounding environment, and a similar degree of plasticity is observed at the levels of cells and tissues. In the intestine of adult Drosophila melanogaster, intestinal stem cells (ISCs) proliferate and differentiate to maintain the populations of absorptiv and secretory cells that comprise the organ. However, the balance between proliferation and differentiation is tipped by external insults such as damage, inflammation, or aging, resulting in an uncontrolled proliferation of stem cells, an accumulation of mis-differentiated cells that maintain hallmarks of both stem and differentiated cells, and a loss of tissue barrier integrity. A signature feature of the mis-differentiated cells is the expression of the transcription factor Escargot (Esg), whose expression in healthy flies is typically limited to ISCs and their immediate daughters, enteroblasts (EBs). Esg is a member of the Snail family of transcription factors that are well-known for roles in development, as well as cancer initiation and metastasis, by regulating epithelial-mesenchymal transitions (EMT). Conserved targets of vertebrate Snail include cell-cell adhesion molecules, such as epithelial cadherin (E-Cadherin). Septate junctions (SJs), the Drosophila analog of vertebrate tight junctions (TJs), between absorptive enterocytes (ECs) and enteroendocrine (EE) cells regulate paracellular flux in the intestine and are required for the maintenance of normal intestinal homeostasis. Compromised SJs in the intestine correlate with increased permeability between the apical and basal epithelial surfaces of the intestine, as well as loss of intestinal barrier function. In Drosophila, Esg is required in ISCs fr maintenance and in EBs to mediate differentiation decisions: loss of esg results in a bias toward the secretory EE lineage. Interestingly, roughly half of the genes known to encode SJ components have been identified as putative direct repression targets of Esg, from a modified chromatic binding assay that detects Esg-binding regions in the genome and from transcriptome analyses of sorted ISC/EBs with varied esg expression. Therefore, the central hypothesis of this proposal is that mis- expression of esg in the intestine in response to damage or age alters the organization of intercellular junctions to disrupt intestinal tissue homeostasis. The proposed study examines this hypothesis through the following specific aims: SA 1) To determine the mechanisms by which esg expression changes in response to aging and stress. SA 2) To elucidate the relationship between Esg and septate junction proteins in the Drosophila intestine. These studies will contribute to the long-term objective in understanding how intestinal tissue homeostasis is maintained with regard to a key transcription program regulated by the Snail family transcription factor Esg and expand our knowledge in the mechanisms by the Snail family can influence tissue homeostasis, impact organ function, and drive tumorigenesis.